A spark plug used for an ignition of an internal combustion engine, such as an automotive engine, generally includes a hollow cylindrical metal shell, a hollow cylindrical insulating body disposed in an inner hole of the metal shell, a center electrode disposed on the leading end side in an axial hole of the insulating body, a terminal disposed on the other end side in the axial hole, and a ground electrode of which one end is joined to the leading end side of the metal shell, and the other end, being opposed to the center electrode, forms a spark discharge gap. Also, a spark plug in which is provided a filler layer wherein an annular space between an inner peripheral surface of the metal shell and an outer peripheral surface of the insulating body is filled with a talc-based filler, in order to prevent a leakage of a gas from a combustion chamber in which the spark plug is installed, is known.
One example of a method of forming this filler layer is described in JP-A-2002-83664 in the following way. “A raw powder manufacturing process whereby raw powder LP is manufactured by combining a specified amount of liquid glass WG and water W with talc powder TP, and mixing and agitating this combination is carried out.” (Paragraph 0027). “The raw powder LP is granulated for an improvement in fluidity, providing granulated filler powder GP.” (refer to Paragraph 0029). “A cavity of a mold 100 is filled with the granulated filler powder GP, . . . , making a compact PC of filler powder.” (refer to Paragraph 0030). “The filler powder is inserted into a space in the form of the compact PC, thus forming a powder filler layer . . . after the insertion of the compact PC, a compression step which compresses the compact PC (powder filler layer) in a direction of the axis of the metal shell 1 is implemented.” (refer to Paragraphs 0034 and 0035). “A caulked portion id is formed by caulking the rear end side rim of the metal shell 1 toward the insulating body while bending it inward by compressing it in the direction of the axis. A seal filler layer 61 is maintained in a compressed condition owing to the formation of the caulked portion id, thus continuously achieving a good sealing property.” (refer to Paragraph 0036).
When forming a seal filler layer, whether with the manufacturing method described in JP-A-2002-83664 or not, it not being possible to form the seal filler layer by using all of initially prepared raw powder with no waste, there exists a large amount of raw powder disposed of partway through the manufacturing process.